A microphone array of this type can for example be used for recordings, where a frequency-independent directivity is desirable. Microphones are inter alia characterised by their sensitivity to different frequencies, but also by their sensitivity to the angle of incidence of the sound waves into the microphone. A microphone may, for example, have a spherical characteristic, where it receives sound waves substantially equally well from all angles, however, a microphone may also have a more or less conical directional characteristic. Thus, the microphone is highly sensitive to sound waves coming from a particular direction and less sensitive to sound waves coming from other directions. When microphones are used for the recording or transmission of, for example, music in a recording studio or a concert hall, the selection of the types of microphones used depends on a number of circumstances, such as, for example, the instrumentation in question, the acoustic environment in the recording room and the desired acoustic pattern. In order to be able to create an optimum recording under a multitude of different conditions, it is required that a large number of different types of microphones is available. Usually, many microphones are used for a task at hand, said microphones being moved around and exchanged with respect to the requirements that may arise. For example, a microphone may be required, where the directivity of said microphone may be improved with respect to existing types, while altering the frequency dependence of the directivity, the basis thereof being a microphone with constant frequency and an improved directivity in a larger frequency range. Thus, the same microphone may be adapted electronically to different needs. The number of different types as well as switching between said types may thus be achieved in a considerably easier and more flexible way.
Further advantages become apparent, if the same microphone could be made to focus on several adjustable directions simultaneously, thus possessing individually adjustable directivity characteristics for each of these directions. Depending on the actual acoustic conditions, such a microphone may replace a varying number of conventional type microphones, at the same time achieving improved results and less time consumption in the recording room.
Thus, there is a need for a microphone with controllable, substantially frequency-independent directivity, i.e. the directivity within a considerable frequency range is substantially the same, or said microphone possessing a preselected characteristic, said characteristic being improved with respect to conventional microphones. It is advantageous that the system is designed in such a way that it is able to focus on several points in space simultaneously.
Systems fulfilling these needs to a varying degree are well-known in the art. U.S. Pat. No. 5,657,393 discloses an elongated microphone array with a plurality of microphones disposed in groups depending on their frequencies, said groups either being disposed adjacent each other or along an elongated array. The system makes use of band-pass filters for each group of microphones, and the resulting signals behind the band-pass filter are summated, and the resulting signal is a signal with high directivity. The system shows a good directivity characteristic in the direction of the elongated array, however, a system of this type is disadvantageous in several ways. The two major disadvantages are instabilities arising at the transition from one group to the next and thus instabilities arising in the frequency-dependent directivity characteristic because of the grouping of microphones according to frequencies. Since the elongated array has a physical extension so that the sound signals reach the individual microphones at different times, a time correlation is used to establish the desired directivity characteristic. A microphone array of this type is often referred to as an “end-fire” microphone.
Joseph Lardies: “Acoustic ring array with constant beamwidth over a very wide frequency range”, Acoustic letters, Vol. 13, no. 5, p. 77-81 discloses a technique for maintaining the beamwidth of a transducer constant over a frequency range of N octaves. An acoustical ring array of six sensors is used to produce a radiation pattern at a given frequency, whereas a half-scale model is implemented to give the same directivity pattern at the double frequency. Compensation filters are used in the respective array outputs to produce a constant beamwidth over the corresponding octave. The design process can be repeated N times in order to obtain an acoustical array with constant beam width over a frequency range of N octaves. However, the beam width is only constant in the plane of the acoustical array. Furthermore, the technique uses eighth-order Butterworth band-pass filters, which have very sharp cut-off frequencies and a flat response in the passband. As a result, the transducer has very distinct sidelopes, which means the directivity of the transducer is very poor. The article does not mention or suggest any means to change the directivity of the transducer.
WO 0158209 discloses a system having a number of microphones disposed in a circle for recording a sound field: The document provides a thorough analysis of the frequency characteristic for such a system and it is shown, how the amplification in the system depends on the number of microphones, and which frequencies are observed. The disclosed examples show a strong frequency dependence with respect to amplitude information, and the system for processing the signals is relatively complicated.
WO 0171687 discloses a surveillance system, where a network of microphones is used to monitor conversations in a large room. A special device is equipped with a large number of microphones in order to obtain high directivity, but this only succeeds at the cost of the frequency information.
U.S. Pat. No. 6,317,501 discloses a system having a network of microphones, said network being used to obtain directional information from incident sound. The system uses filters and time delays to generate an output signal. The system is specifically designed to find directional information in a sound field.
U.S. Pat. No. 6,526,147 describes an elongated microphone array with pairs of microphones disposed on each side of the microphone array. The microphones are arranged equidistantly. The signal from each pair of microphones is summated and transmitted to a filter, and the resulting filtered signals are summated. However, the results shown display a certain frequency-dependent directivity.
U.S. Pat. No. 4,696,043 shows a microphone array with microphones disposed equidistantly, a network with weighting factors being used to alter the directivity characteristic of the system. It is shown that a great number of different directivity characteristics are obtained, however, said characteristics are highly frequency-dependent.
U.S. Pat. No. 5,058,170 discloses a directional microphone array provided to suppress acoustic feedback and howling generated by loudspeaker systems.
U.S. Pat. No. 5,473,701 discloses a system for use with mobile telephones, where two microphones are used to obtain high directivity. This is achieved by means of inter alia delay circuits and low-pass filters.
US Patent Application No. 20020069054 discloses a system having a number of microphones, said microphones apparently rotating in space by means of time delays. The document also states that the system can focus on several points simultaneously.